Thriving Colonies of Rare Mollusks Discovered in the Rio Grande

A thriving colony of a rare mollusk species was discovered in Texas in early 2011 during fieldwork led by scientists from the Great Lakes Center at Buffalo State.

Lyubov Burlakova, project director and research scientist with the center, and Alexander Karatayev, director of the center, explained that the discovery was important because the species is considered to be at high risk of extinction.

Popenaias popeii, commonly called Texas hornshell, is a species of the Unionidae family of freshwater bivalve mollusks. It is endemic to the Rio Grande watershed. Endemic species are species that are native to, and can only survive in, a specific habitat within a particular region. As far back as 1893, researchers considered the Texas hornshell to be rare. In 2010, the Texas hornshell was added to the Texas list of threatened species; it is currently a candidate for listing under the Federal Endangered Species Act.

Burlakova and Karatayev, with a team of scientists from New Mexico, Ohio, and Texas, initiated a mark-recapture study of the Texas hornshell to assess the population’s status and to monitor it in the future. Because the newly discovered colony of Texas hornshell is healthy and reproducing, prompt conservation actions may ensure the survival of the species.

Burlakova and Karatayev, international experts on freshwater mollusks, received initial funding for conducting research on Texas mollusks in 2004, when they were affiliated with Stephan F. Austin State University in Nacogdoches, Texas. During earlier research expeditions, conducted with Thomas Miller, director of the Environmental Science Center at Laredo Community College, they found isolated specimens of the Texas hornshell. One goal of this year’s research was to identify its specific habitat.

“We found that perhaps 8,000 hornshells live in the area near Laredo where we found the colony,” said Karatayev. “But this two-mile stretch of the Rio Grande is the only place in the world where we know such a large colony exists.”

One characteristic of endemic species is that their survival depends on a complex habitat comprising specific elements. “For example,” said Burlakova, “this mollusk requires certain fish species for their larvae to survive.” The scientists discovered that Texas hornshells also require a very specific habitat: groups of large boulders that provide stable, low-flow refuges where the hornshells can live in clay seams under the boulders.

Scientists recognize the Texas hornshells and other species belonging to the Unionidae family as important components of aquatic ecosystems, and as indicators of water quality. They improve clarity and quality of water by removing algae and suspended matter by filtration, and their shells provide shelter and habitat for bottom fauna. However, the mollusks’ long life span, slow growth and reproductive rates, sensitivity to environmental disturbance and pollution, and their dependence on particular species of fish to ensure the survival of their young combine to make unionids the most imperiled group of animals in North America. More than 70 percent of unionid species are threatened, endangered, or of special concern.

“Our survey showed that Texas hornshell is still viable in the Rio Grande,” said Burlakova. “We need to do all we can to ensure its survival.”

Members of the scientific team that took part in the 2011 Texas fieldwork are B. Lang, New Mexico Department of Game and Fish; D. Berg and K. Inoue of Miami University in Oxford, Ohio; T. Miller, Environmental Science Center at Laredo Community College; and Y. Zhang and T. Noble of Texas State University-San Marcos.